Solid state starters/controllers have found widespread use for controlling application of power to an AC induction motor. The conventional starter/controller, referred to hereinafter as simply a soft starter or a controller, uses solid state switches for controlling application of AC line voltage to the motor. The switches may be thyristors such as silicon controlled rectifiers (SCRs) or triacs.
One application for a motor controller is as an elevator starter. The elevator starter may be used to drive a pump for an hydraulic elevator. Each time movement of an elevator car is commanded, then the starter must start the motor until it reaches operating speed and then operate in a run mode. Such a starter may only be used for the up direction as gravity may be used for the down direction.
Most industrial and commercial establishments have three phase power readily available to power three phase motors and equipment. In situations where three phase power is not available, or cannot be made available, converters are often used to convert single phase power to three phase power. There are different types of single phase to three phase converters. One type is a rotary phase converter. A rotary phase converter includes a capacitor bank that in conjunction with a single phase supply starts a three phase motor to provide the third phase. Once the three phase motor achieves a threshold speed, the single phase supply, along with the generated third phase supply is the three phase voltage. The capacitor in this configuration is only used for starting. The use of a rotary phase converter requires operation of a motor to run another motor. This can be inefficient.
Another type of single phase to three phase converter, known as a static phase converter, uses capacitors to cause a phase shift in one of the single phase lines to create the third phase. When the converter is energized, the load must draw current to achieve the voltage shift in the capacitors required to enable a phase shift.
A typical soft starter requires an acceptable three phase supply before the SCRs are fired to provide power to the load. For systems that use a rotary converter, the soft starter must wait for the three phase motor portion of the converter to achieve a threshold speed required to provide an acceptable three phase supply for the soft starter to run the motor.
For systems using a static converter, at startup the soft starter receives single phase power which is not acceptable. Unless the soft starter receives an acceptable three phase supply, it will not gate the SCRs. Likewise, unless the soft starter gates the SCRs, there will not be an acceptable three phase supply. Additionally, some applications require a reverse phase, or voltage monitor to ensure that the voltage is in the proper rotation before allowing the motor to start. Under the circumstances described above, the protective relay will not allow the motor to start unless there is acceptable three phase power. Likewise, unless the motor is allowed to start, the static converter cannot produce three phase voltages.
As of the result of problems associated with conventional systems and methods for operating a soft starter with a single phase to three phase static converter, mechanical starters which include contactors are generally used instead of a soft starter. However, while a mechanical starter can allow a motor to start and run, mechanical starters do not provide the current control, fault detection, and motor protection that a solid state device such as a soft starter provides.
The present invention is directed to solving one or more of the problems discussed above, in a novel and simple manner.